Currently, loudspeaker systems commonly utilize a multispeaker approach in which two, three, four or more speakers are used in a single loudspeaker or system. These systems have been developed in order to provide a sound producing device which accurately and efficiently produces different types of sounds having a wide range of frequencies. For example, such multispeaker arrangements have been developed in order that such systems or loudspeakers produce voice or speech and music equally well.
These systems often include one or more of a variety of horn and/or diaphragm speakers torn and diaphragm type speakers both utilize a driver connected to a diaphragm. The horn type speaker further includes a horn or waveguide connected to the diaphragm, however, for transmitting the sound generated by the diaphragm. The waveguide normally comprises a throat and a mouth connected by a passage. In the horn type device, the diaphragm is located at the throat of the waveguide, and the passage directs the acoustic energy to the open end or mouth of the horn where it emanates to the listener.
In designing loudspeaker systems utilizing horn or diaphragm only type speakers, a number of goals are important. First, it is desirable that the loudspeaker efficiently produce the sound, requiring as little power as possible. Second, it is desirable that the speaker be as compact as possible for its intended use, without otherwise sacrificing sound reproduction characteristics. Third, the speaker should produce and emanate sound directly to the listener, i.e., the sound waves generated by the speaker should not be blocked or interfered with by the speaker itself as the sound waves leave the speaker and radiate towards the listener. Fourth, it is desired that when using multiple radiating devices together, that all the sounds leaving the devices are in phase. Lastly, the speaker should exhibit good directivity characteristics. Directivity in the speaker context refers to the ability or the speaker to emit waves which are concentrated. The result of good directivity is the ability of the listener to discern the direction from which the sound is emanating because sounds from other directions are attenuated. In large enclosed spaces, such as large rooms, increased directivity means that the amount of direct sound, as opposed to reflected sound, which reaches the listener is increased. In horns, directivity is directly related to the geometrical shape of the waveguide.
Prior to this time, there have been many attempts at designing a loudspeaker which has each of the desired characteristics. Each of these designs, however, suffers a problem which, before the invention later described, has not been solved.
First, a number of loudspeakers have been designed which utilize a horn-in-horn approach. These speakers were designed primarily in an attempt to reduce the size of the bulky multispeaker systems described above. In this type of loudspeaker, a large horn is designed to produce lower frequency sounds. Normally, a low frequency driver is connected to this large horn, with the waveguide directing the sound from the driver to a mouth located at the front of the enclosure. A smaller horn designed to produce higher frequency sounds is located within the waveguide of the larger horn. This horn utilizes a high frequency driver connected to a waveguide which is much smaller than that of the larger horn. The high horn waveguide is normally mounted to the walls of the waveguide walls of the low horn, near the mouth of the waveguide, utilizing some type of bracket.
The horn-in-horn design suffers, however, from numerous drawbacks. First, the placement of the small horn directly in the waveguide of the large horn creates an acoustic shadow. This shadow represents low frequency sound waves which are blocked as they emanate from the large horn by the small horn which is located in the low horn waveguide. Further, the placement of the drivers of the small horn in front of the drivers of the large horn means that the sounds from these two horns are produced in two different locations. As the sound leaves the speaker, the high frequency sounds and low frequency sounds are thus out of phase because of the distance between their source points. In order for the sound to accurately be reproduced, an appropriate signal delay or other circuit must be added in order to delay the electrical signal routed to the high horn, in order that the sound from the small horn be delayed such that the sound which leaves the loudspeaker is in phase.
This loudspeaker design also suffers from a directivity problem. As stated above, waveguide geometry affects the directivity or sound coverage of the sound leaving the horn. However, at a given frequency, a corresponding waveguide shape is necessary to maximize directivity. Because each horn produces sound over a wide bandwidth, the waveguide for each horn is normally designed to maximize directivity with respect to sound in the entire bandwidth. Therefore, the horn geometry is usually chosen so directivity is maximized for sound in the middle of the bandwidth, with the directivity for sounds having frequencies on either end of the bandwidth being less than optimum. The largest loss in directivity, however, occurs at the cross-over point between the two horns. At this point, sounds having frequencies only a few Hertz apart are produced by horns having very different geometries. This means that essentially the same sounds are created by horns which act very differently upon the acoustical waves. Therefore, the directivity at these crossover points is compromised. In multiple horn loudspeaker systems, especially those in which the cross-over frequency is in the vocal band, the loss in directivity causes the sound coverage to change dramatically over this frequency region.
Several loudspeaker designs have attempted to improve upon the above design. One such system is described in U.S. Pat. No. 5,046,581. This patent describes a horn-in-horn loudspeaker system in which the drivers of the small horn are located the same distance from the mouth of the small horn as the low drivers are from the mouth of the large horn. This is accomplished by having the drivers of the small horn connected to an extremely long passage which leads to the small horn waveguide. This design thus eliminates the phase problem and eliminates the need for expensive signal delay circuits. This design, however, still suffers from shadowing and directivity problems.
Several loudspeakers have also utilized a horn for producing high frequency sound located inside of the diaphragm of a low frequency producing speaker. One such design is described in U.S. Pat. No. 2,269,284. This patent generally describes a loudspeaker in which the driver of the horn and the diaphragm speaker are concentric, thus eliminating phase problems. This design, however, still has severe directivity problems. While the walls of the horn and diaphragm are essentially one and the same, thus limiting directivity problems at the cross-over frequency in the abstract, several other problems are created. First, the shape of the diaphragm is not well suited for use as a waveguide, meaning that overall directivity is lost for sounds created by the horn. This loss cannot be fixed by altering the shape of the diaphragm either, for once the diaphragm is so altered, it no longer functions to produce the low frequency sounds. Further, because the diaphragm is not well suited for use as a waveguide, part of the efficiencies normally gained through use of the horn are lost.
Lastly, U.K. Patent No. 303,837 describes a loudspeaker utilizing two horns whose sound channels merge to form a single outlet. This design eliminates shadowing and phase problems, but suffers from several other disadvantages. First, this design is a line array type arrangement, thus causing the sound from each single driver to remain a separate source. Second, this arrangement of the sound channels does not permit the sound from both sources to be coincident in both the horizontal and vertical plane. Thus, true point source performance is not achieved. Lastly, the throat size at the merger of the sound channels cannot be simultaneously large enough to allow passage of the low frequency sound without coloration, and yet at the same time be small enough to allow proper diffraction of the highest frequencies.